This Specialized Center of Research (SCOR) Program is a collaborative effort of three institutions in Wisconsin, namely the Medical College of Wisconsin, Milwaukee; the University of Wisconsin Medical School, Madison, Wisconsin; and the Marshfield Clinic Foundation, Marshfield, Wisconsin. The goal of this program is an in-depth investigation of a number of aspects of hypersensitivity pneumonitis, a disease characterized by the development of respiratory and systemic symptoms following sensitization and re-exposure of susceptible individuals to a variety of organic dusts. The program is directed toward studying the epidemiology, natural history, clinical features and pathogenesis of these diseases.